residential stray voltage - investigation procedures

[magoo2]

I'm interested in seeing what information may be out there on utility procedures for investigating stray voltage complaints from residential customers. This is mainly involving 120/240 V single phase service from single phase transformers, so its really North American practice.

Common situations involve people getting shocked in a shower.

Most of what I've found relates to dairy farms and milking equipment. Also there's some dealing with swimming pools, but I'm really not interested in that.

 

[itsmoked]

Common?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PHovnanian]

The utility will (may?) check their connections bbetween the transformer and point of service (weatherhead, meter base). Then, they'll advise the customer to contact an electrician.

The symptom you describe is usually the result of a bonding or grounding problem in the customer's system. Not something a utility cares to get involved with.

 

[magoo2]

Common was a bad choice of words on my part. Given that you have a customer that calls in about getting shocked, it is commonly identified as someone getting shocked in the shower. However, this is a fairly low probability event.

We'll typically pull the meter at the residence and if the neutral to ground voltage drops to zero then we'll conclude that the problem is on the customer's side and then advise him to get an electrician.

 

[waross]

I encountered a "hot" floor. A staple had been driven in too hard and had crushed the insulation on the Romex and made contact with the hot wire. I could read 120 volts with the meter probe on the wood near to the staple.
On the wooden floor above, I could read about 40 volts. When someone tried to drink out of the tap, they got a painful shock in the mouth.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

magoo2; Your "pull the meter method" is a good one.

Beyond that it becomes a regular hunt with a meter and some knowledge. I generally plug in a grounded extension cord somewhere distant, drag the other end over to the problem area and then using the tested,(hot to ground), ground socket as the ground anchor and a meter hunt down the problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lansford]

If the issues is not related to a problem in the house wiring, and the utility finds that a neutral to earth voltage is an issue they will take remedial actions. One of the most common causes is unbalance phase current which can show up as neutral to earth voltage. Re look at the problem relating to dairy farms because it it the same concept that can happen in a residence.
JIM

 

[racookpe1978]

OK.

So how am I am getting very. very low voltages on my "dead" AC 120v circuits? (Digital voltmeter reading 2-3 volts on isolated circuits. That seems too high for just random readings, but too low for any short in the ground side.)

 

[wayne440]

(Most) digital meters are very high impedance, and will show low ACV readings via inductance, seemingly out of thin air. If you get a Simpson 260 or similar meter and check your "dead" 120v circuits, there is a better than 99 percent chance they will read 0V.

 

[racookpe1978]

Thank you.

 

[BJC]

magoo2
I have seen the problem and it was the results of bad grounds at the utility transformer (pad mount single phase 7200-120/240).
The particular installations I looked at had a ground rod driven at the transformer. The service neutral and the concentric ground on the 7200 volt cabel were connected to the ground rod.
The concentric neutral around the primary cable provided more of the ground connection than people thought. As the concentric neutral corroded stray voltages appeared. Usually it was people getting tingled when touching the sink etc.
The same problem appeared when primary cabels were repalced with cables routed in Polyethelyne conduit ( directional drilling projects). The cables in the Poly pipe lost a lot CN ground connection.
The soulution was to suppliment ground rods with copper plates. They were 24" x 12". They worked most of the time, the area was one of the wettest in the US. In dry summers somtimes the proble would resurface.

 

[busbar]

Wayne440, for voltage measurement with hi-Z meters, check out the Fluke TL225 Stray Voltage Adapter at fluke.com.

 

[rockman7892]

"If the issues is not related to a problem in the house wiring, and the utility finds that a neutral to earth voltage is an issue they will take remedial actions. One of the most common causes is unbalance phase current which can show up as neutral to earth voltage. Re look at the problem relating to dairy farms because it it the same concept that can happen in a residence."

I would imagine that unbalanced phase current would only cause an issue if the transformer neutral was not solidly grounded. If transformer was grounded then neutral would be held to ground potential, and I dont see how unbalanced current would cause issues with Neutral to ground readings.

 

[Compositepro]

Residential service must ground the neutral at the service panel so I would think that any grounding problems are not the utilities responsibility.

 

[resqcapt19]

I don't think just pulling the meter and measuring the neutral to ground voltage tells you much. If there is a poor connection on the neutral on the line side of the meter, that problem won't show up unless you measure the voltage with current on the neutral. A poor connection on the neutral between the utility transformer and the serice main bonding jumper will impose a voltage on the building grounding system and everthing connected to it. Other than a metal underground water piping system that is common to other buildings in the area, the grounding electrode sytem at the building does not have a low enough impedance to prevent a shock hazard. In the case of a poor service neutral and metal interior water piping, the voltage drop on the neutral, as a result of the poor connection, will appear on the water supply piping system. This can result in a shock hazard to the drainage system as that system is not normally bonded. That being said a bad neutral will normally present other symptoms, the most common of which is high and low voltage across the connected loads.

 

[magoo2]

resqcapt19,
You're on to step 2.

Initially someone is getting shocked. Pulling the meter will help determine if the fault is on the customer side of the meter or not. Since we run into a lot of wiring issues, this is an important first step.

Step 2 is to put what is called a beast of burden on the meter base. This a single phase heater load that can be switched between legs of the service. This will check the condition of the service neutral as well as test for an open hot leg.

 

[Robert789]

A large percentage (maybe over 50%) of the instances I've seen involving someone being shocked in a shower ended up being due to a shorted element in an electric hot water heater. Often in the same residence where the problem was reported, but frequently several houses away.

When we receive complaints of people being 'shocked' in swimming pools (usually, its a woman with recently shaved legs experiencing a tingling sensation when touching a ladder), it is usually due to neutral currents returning through the earth. I haven't seen this in any swimming pools that are bonded in accordance with the requirements of the 2007 NESC.

Robert

 

[resqcapt19]

Robert,
Are the pool bonding rules in the NESC different from those in the NEC? Most pools are installed under the rules in the NEC.

 

[magoo2]

I'm sure he meant to say NEC for pool bonding.

 

[Robert789]

Sorry about that, the applicable code is the NEC (rules for bonding of pools are in section 680.26), not NESC. I think that this issue was first addressed in the 2005 version and bonding requirements were again strengthend in the 2008 version.

Robert